In a number of applications, particularly in the automotive industry, there is a need for light-weight, high-strength structural members. Although structural members having these characteristics can be readily obtained through the use of various metal alloys such as titanium alloys and the like, light-weight, high-strength alloys are generally cost prohibitive in automotive applications where weight reductions are closely balanced against the cost of materials. Moreover, reinforcement techniques are required which can be readily adapted to existing geometries of structural parts, thereby eliminating the need for fundamental design changes and providing a means by which substandard design performance can be remedied. That is, in many instances design deficiencies are discovered after vehicle design has reached the stage at which radical changes are no longer feasible.
In addition, a significant amount of emphasis has been placed on the performance characteristics of channel-shaped structural components which encounter forces that produce bending. For example, many side impact beams designed for motor vehicle doors have a channel-shaped cavity. In addition, many functional bumpers are channel-shaped. These channel-shaped sections are most susceptible to bending forces which originate at or concentrate in the midspan of the beam.
Although filling the entire section with plastic foam does significantly increase section stiffness (at least when high-density foams are utilized), this technique may also significantly increase mass and thus part weight, which, as stated, is undesirable in most applications. In addition, filling a section entirely with foam can contribute significantly to cost. Finally, a large foam core often creates an unwanted heat sink. And, although increasing the metal gauge of a section or adding localized metal reinforcements will increase stiffness, as the metal thickness increases, it becomes more difficult to form the part due to limitations of metal forming machines.
A number of approaches have been proposed for dealing with the problem of reinforcing channel-shaped sections subjected to bending as alternatives to high-cost alloys, thick-metal sections and large foam cores. For example, a side impact beam for a vehicle door has been proposed which comprises an open channel-shaped metal member having a longitudinal cavity which is filled with a thermoset or thermoplastic resin-base core. The core is disposed in the midspan of the beam. The core may include hollow glass microspheres in order to decrease density and thus weight.
A reinforcement insert comprising a precast reinforcement has been proposed. The reinforcement is formed of a plurality of pellets containing a thermoset resin and a blowing agent. The precast member is expanded and cured in place in a structural member. A composite tubular door beam reinforced with a syntactic foam core localized at the midspan of the tube has also been described in the art. The resin-based core occupies not more than one-third of the bore of the tube.
Tube-in-tube structures having high stiffness-to-mass ratios have also been proposed in which two nested tubes have a layer of foam disposed in the annulus between the tubes. A local reinforcement in the nature of a foamable resin disposed on a drop-in carrier has also been described. The carrier is placed in the channel of a hollow structural member following which the resin is expanded.
Accordingly, it would be desirable to provide a low-cost technique for reinforcing a channel-shaped section subjected to bending without significantly increasing the mass. It would also be desirable to provide a method of reinforcing an existing channel-shaped section which does not require any fundamental design change to the member. The present invention provides hollow sections which have increased strength with moderate increases in mass, all without the use of high volumes of expensive resins. The present invention further provides a method for reinforcing existing structural parts without redesigning the geometry of the part. It has been found that the present invention increases section stiffness and strength in channel-shaped sections in a highly efficient manner.